The RED Machine

A rotor machine usually uses a disk with 26 contacts arranged
in a circle on each side. When the rotor moves, the contacts on
both sides of it advance.

A rotor which had 26 contacts in a circle on one side, and a spindle
with 26 contact strips on the other side, is always sure, when it moves,
to take each letter to a different substitute. Such a rotor is called
a half-rotor, and the Hagelin B-211, which we will meet later, uses
such rotors as well.

The Japanese cipher machine which was codenamed RED by American
codebreakers behaved as if it consisted of two half-rotors, one with
20 contacts for 20 consonants, and one with 6 contacts for 6 vowels,
counting Y as a vowel. This property allowed encrypted text to be
pronounceable, thus allowing use of the commercial telegraph system.

This division of the alphabet was perpetuated in the PURPLE machine,
which was a weakness of that machine, although it at least had a plugboard
so that the six letters handled by themselves could be any six letters.

Also, while American codebreakers built their own RED work-alikes
that really did have two half-rotors, the actual machine used by the
Japanese had only one half-rotor; the spindle had all 26 strips, and
the wheel face had two sets of 60 contacts, one wired with the same
scramble of 20 characters repeatedly, and the other wired with the same
scramble of the other 6 characters repeatedly.

The 60-contact wheel usually moved one step for each
character enciphered, but not always. Its motion was controlled by
a 47-position gear or pinwheel. Only eleven of the teeth could be
disabled, including four adjacent pairs of teeth.

When an enabled tooth is active, the rotor advances one
position for the letter currently enciphered.

When a disabled tooth is encountered, the rotor also advances one
position, but the 47-position gear continues to advance for the current
letter. Thus, occasionally the rotor will move two or even three
positions when a letter is enciphered, but it will always move exactly
47 positions for each cycle of the 47-position gear. That cycle
will take less than 47 letters, one less for every disabled tooth.